When it comes to the power source, I am a big fan of the AA battery lights. In the past the biggest reasons for the use of lithium CR123 batteries instead of AA batteries were more consistent output, no leakage, and long shelf life. But current AA batteries and the flashlights powered by them seem to have no problems with consistent power. In fact, there are lithium AA batteries cheaper than CR123s, and Eveready is advertising leakproof alkaline batteries. As for shelf life, AA batteries from reputable makers now can be stored for as much as 10 years. And one of the reasons that I really like AA batteries in mission critical lights is that when you find yourself somewhere in need of batteries you can find AAs just about anywhere, even in a TV remote.
When it comes to the controls on my flashlight, I just want the basics, and I think most cops feel the same way. Obviously your handheld light needs a momentary on/off switch that also allows you to click it in the on position; everything else isn't as important. An ability to have a low setting is nice for taking notes, and it's becoming more important with the higher lumen output lights. But it's important that it not accidentally give you low light when you want high.
Then there's the strobe. Most police flashlights have this feature now, but I have not seen any regular working police officers use it. Yes, it disorients the suspect, but it also disorients you and other officers. So I think it is important that flashlights that have this feature give you an option to lock it out. Before I get hate mail, I have really tried to get the strobe function to work for me in some kind of situation. I even took an early tactical light with a strobe feature out in the field and used it on suspects, but I got tired of being disoriented myself. Maybe it's personal preference, but that was my experience.
Many officers believe in the old tactical axiom that in order to make sure you have one light, you need two, since Murphy's law can bite you when you least expect. This is one reason why small AAA-powered lights are popular in law enforcement. Some of the newer AAA lights put out more than 65 lumens so they can make a good backup light.
Another reason for the popularity of smaller lights is they can be really useful when taking notes or cite writing. Some of them have a clip that allows for bezel down carry. This is an excellent feature. I have found that you can clip it to your shirt pocket or at the opening of your uniform shirt.